Use a font like Verdana or Arial. These are called "sans-serif"
fonts, meaning they don't have little points at the ends
of each letter like Times Roman. I use -1 and -2 sizes.
This enables peoples to change the text size in their browser
if the prefer larger or smaller text.
This is Verdana abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ.This is Arial
abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ.

"My interest in usability arose from the pain
and tears of patching the wounds of suffering interface designs
with the inadequate bandages of help files and user guides."
— Daniel Cohen